This invention relates to the pasteurization of dairy products such as milk, cream, and ice cream mix and other liquid food products, e.g., fruit juices and soups. The invention is more particularly concerned with a continuous-process pasteurization technique for heating milk (or other food product) to an ultra high temperature suitable for long shelf life, and which recovers most of the heat employed by regeneration, and where the pasteurized product is cooled by incoming raw product, which in turn is heated to a temperature near the pasteurization temperature. The invention is also concerned with a technique which avoids a cooked or scalded taste in the food product.
Unprocessed milk and other raw food commodities are highly perishable, and also may harbor pathogenic microorganisms. Milk is a highly nutritious food, and thus also serves as an excellent growth medium for microorganisms, most of which are capable of deteriorating or spoiling the milk or milk products. This is the case also for fruit juices, broths, and many other liquid food products. Some microorganisms are pathogenic, and can present a threat to public health, and so it is important that the product is rendered completely free of pathogenic microorganisms. Therefore, milk and other liquid food products are processed to make them safe for human consumption and to reduce spoilage to the lowest level possible.
Pasteurization is a process for heat treating the milk or other food process to kill these microorganisms. For milk, pasteurization requires that every particle of milk or milk product be heated to a temperature, for an adequate length of time sufficient to render it free of pathogens, most notably Rickettsia and Mycobacterium tuberculosis. The U.S. Public Health Service has published standards for time-temperature relations for pasteurization of milk and other milk products, and these can be at higher temperatures or longer times for products that may contain added sweeteners or additional milk fat. Most modern dairies employ a continuous process pasteurization technique, rather than a batch process, even for highly viscous products such as ice cream mix. Typically, for most pasteurized milk, other than ultra-pasteurized or UHT pasteurized, a continuous process high-temperature, short time (HTST) technique is used, in which the milk is heated to about 165 degrees F and then passes through a holding tube where the milk is held at that temperature for a short time, e.g., sixteen seconds. These HTST pasteurizers can employ either a plate heat exchanger, or “press,” in which parallel plates define flow channels for the milk and for heating and cooling media, or can employ a tubular heat exchanger in which two or more tubes of different diameter are arranged coaxially to define flow paths for the milk and other heat transfer media.
In a HTST set up, cold raw milk (about 40° F.) is supplied from a balance tank into a regenerator section of the pasteurizer. Here the raw milk is heated up by heat given up by pasteurized milk flowing in counter current direction through an opposite side of the regenerator. The raw milk leaves the regenerator and passes through a positive displacement timing pump that delivers the milk under pressure to further stages. The pre-heated raw milk is then forced through a heater section where a heating medium, e.g., water or a food grade synthetic product, heats the milk up to a temperature of at least about 162° F. The milk, having reached this pasteurization temperature, flows through a holding tube or timing tube, where the milk is held at this temperature for a predetermined time, e.g., at least 16 seconds. The velocity of the milk product is determined by the speed of the timing pump, the diameter and length of the holding tube and other sources of surface friction. After passing temperature sensors at the end of the holding tube, the milk flows past a flow diversion device, which is intended to return the milk product through a divert line to the balance tank if the temperature of the product is below the preset pasteurization temperature. Properly heated milk will continue forward, and passes through the pasteurized side of the regenerator where it is cooled by the incoming raw milk. The milk can be cooled further to about 40° F., and processed for bottling, packaging, cheesemaking, or other use. A homogenizer may be used as the timing pump just described, or may constitute additional auxiliary equipment, typically at the stage where the milk or milk product has been heated. Booster pumps are also present in the flow path to ensure correct pressure and flow relationships. For any continuous pasteurization technique, it is important to maintain a higher pressure on the pasteurized side of any product-to-product heat exchanger, such as the regenerator, so that in the event there is any pinhole leak the flow of milk is away from the pasteurized side. This prevents contamination of the pasteurized milk with raw milk. The pressure differential is maintained using the timing pump, and other pumps and pressure controllers, and by ensuring that there is any vertical rise in the product flow path is kept within limits.
The purpose of the regenerator is to save energy used in heating and cooling the food product during pasteurization, by using the heat content of the pasteurized product to pre-heat the incoming cold raw milk. The efficiency, i.e., percent regeneration, is equal to the ratio of the temperature increase in the raw milk due to regeneration to the total temperature increase. For example, for cold milk drawn from the balance tank at 40° F., heated through the regenerator to 145° F., and then heated to a final temperature of 165° F., the efficiency would be 84%, i.e., an 84% regeneration:
            (              145        -        40            )              (              165        -        40            )        =            105      125        =          84      ⁢      %      
As the cost of energy is a significant consideration in the overall cost of processing the milk product, it is desirable to keep the amount of regeneration as high as possible, and thus to reduce the cost of adding heat at the heater stage.
Ultra high temperature treatment of the food product, i.e., UHT pasteurization, involves heating the liquid food product continuously, and ensuring that every particle of the milk or other food product has been held at the predetermined ultrahigh temperature for a minimum length of time. The UHT technique can be incorporated into a sterilization technique, in which the product is heated to a temperature of 240° F. or above, and is held for a corresponding holding time to ensure that the microorganisms and their spores in the product are destroyed. Then the sterilized product is packaged aseptically, and aseptically sealed. The intention here is to permit the liquid food product to be stored at room temperature indefinitely without spoilage due to action of microorganisms. However, the process of ultra high pasteurization processing may alter the flavor or desirable color or texture of the product, and may result in a “cooked” or scalded flavor in the product.
A vacuum treatment is sometimes employed to remove as much of the undesirable flavor components as possible from the product. In the vacuum process, milk is first heated to the desired temperature, and then is passed into a chamber in which the pressure has been reduced by a partial vacuum. The pressure in the chamber is low enough to cause the volatile flavor components to vaporize, and these are then evacuated from the chamber. Some of the water in the product may be evaporated as well. The vacuum treatment will also reduce any flavor components that result from the cows' ingestion of weeds or flavor-producing feed components.
In addition to the public health and spoilage issues addressed by pasteurization, it has been discovered that some proteins and enzymes in milk that will bring about chemical changes in the stored product can be altered or removed, i.e., denaturized, by heat treatment.
A continuous flow milk sterilization process is described in U.S. Pat. No. 3,567,470, in which raw milk is passed though counterflow regenerators to a pasteurizer and then is passed though counterflow heat exchangers where its temperature is raised to a sterilization temperature. However, in this process, there is a rather high temperature differential between the milk and the heating medium in the ultra high temperature heat exchangers used for sterilization, so that the temperature differential between the milk and the heating medium at any given point of reference is always above 20 degrees and may reach 40 degrees F. This means that the heating medium (steam) entering the heat exchanger has to be 300° F. to raise the milk to a sterilization temperature of 270 to 280° F., and the medium leaving is at 280° F. where it meets the product entering at 240 to 260° F. The processed milk in this system has to pass through a pair of vacuum tanks to remove air and entrapped volatile gases in an attempt to remove the objectionable burnt flavor that is associated with the sterilized milk.